A pattern of hybridisation and population genetic structure of two water frog species (Ranidae, Amphibia) in the southwestern Balkans

The distribution of the neutral component of genetic diversity is the interplay of historical and ongoing processes resulting in the species-specific genetic structure of populations, which can, however, be disrupted by interspecific hybridisation and introgression. In this study, we focused on two species of water frogs, Pelophylax epeiroticus and P. kurtmuelleri, which live in sympatry in the southwestern Balkans, to investigate the rate of hybridisation and population genetic structure using cytogenetic, mitochondrial (ND2) and nuclear DNA (microsatellite) markers. The overall hybridisation rate was 2.6%, with rates reaching up to 10% at specific sites. The course of gametogenesis and the occurrence of later generations of hybrids (beyond the F1 generation) indicate a sexual mode of hybrid reproduction. The bimodal structure of hybrid populations and the rarity of hybrids suggest substantial reproductive isolation between the two species; however, this isolation is unlikely attributable to differences in ecological niche occupation. In P. epeiroticus, sequence variation in the ND2 gene revealed two divergent lineages with a clear geographic pattern that corresponds to the genetic structure in microsatellite markers. In contrast, P. kurtmuelleri populations were not as geographically structured and showed only weak genetic differentiation in both types of markers. Pelophylax epeiroticus was significantly less variable at microsatellite loci compared to P. kurtmuelleri, which, together with the high differentiation of its populations, suggests a stronger influence of genetic drift. We can hypothesise that the differential strength of genetic drift in the two species may lead to unequal interspecific gene flow.